Interpretive Summary: The glassy-winged sharpshooter (GWSS) appears to be limited to discrete regions within California where winter temperatures are mild and the temperature rarely drops below freezing. Laboratory and field studies indicated that GWSS adults cannot feed at maximum daily temperatures below 50ºF (=10ºC), thereby reducing its ability to survive cold winters. We verified the impacts of cool temperatures on GWSS adults by exposing them to a regime of seasonal temperatures (within temperature cabinets) that reflect a variety of areas within the state and confirmed that periods of temperatures above the threshold for feeding was a critical factor in determining adult feeding and survival. In a field experiment, we validated a developed mathematical model that accurately predicted the temporal pattern of overwintering mortality of H. vitripennis adults at Bakersfield and Riverside, CA. Results indicated that GWSS adults would experience about 92 percent overwintering mortality prior to reproduction in the spring. The potential for temperature-based indices to predict temporal and spatial dynamics of GWSS overwintering and a more complete understanding of the climatic factors that influence GWSS populations may result in the development of strategies to focus control efforts, enhance the efficacy of biological control, and effectively limit the spread of Xylella fastidiosa induced diseases to susceptible crops.

Technical Abstract:
The glassy-winged sharpshooter, Homalodisca vitripennis (Germar), is an invasive species that vectors the bacterium Xylella fastidiosa that induces Pierce’s Disease of grape. This study determined the effect of temperature on the feeding activity of H. vitripennis adults measured as the amount of excreta. The Logan Type I model described a typical nonlinear pattern showing that excreta production increased up to an optimal temperature (33.1°C) followed by an abrupt decline near the upper threshold (36.4°C). A temperature threshold for feeding, at or below which adults cease feeding, was estimated to be 10°C using a linear regression model based on the percentage of adults producing excreta over a range of constant temperatures. A simulated winter-temperature experiment using fluctuating thermal cycles confirmed that periods of temperatures above the threshold for feeding was a critical factor in determining adult feeding and survival. Using data from the simulated-temperature study, a predictive model was constructed by quantifying the relationship between cumulative mortality and cooling degree-hours. In field validation, the model accurately predicted the temporal pattern of overwintering mortality of H. vitripennis adults at Bakersfield and Riverside, CA, in the winter of 2006-2007. Model prediction using temperature data from a Riverside weather station over 9 winters (1998-1999 through 2006-2007) indicated that H. vitripennis adults would experience about 92% overwintering mortality prior to reproduction in the spring, but levels of mortality varied depending on winter temperatures. The potential for temperature-based indices to predict temporal and spatial dynamics of H. vitripennis overwintering is discussed.